1. Field of the Invention
The present invention relates to color printers and, more particularly a high speed thermal color printer for combining sequentially applied monochrome images into a full color image in a single pass of the medium.
One available process (dye diffusion thermal transfer or "D2T2") uses a ribbon which is impregnated with a dye that can be made to diffuse into the surface of a medium. Because this method depends on the diffusion of the dye into the medium, it is a relatively slow process and high speed media movement is not a requirement given the limitations of presently available dyes.
A second process, approximately ten times faster, uses colored wax "ribbons" (thermal wax transfer process or "TWT"). At the present time, full color printers are available that work in the thermal wax transfer ("TWT") process in which a print head having a plurality of individually addressable electrodes that can be selectively heated transfers dots of wax from a ribbon to a medium, usually paper. Such printers are generally designed to work at a print density of up to 400 dots per inch.
Complete images in full color are created by sequentially depositing colored wax dots in complete or partial superposition such that several colors can be created, much in the fashion of multicolor impression printing in which several engraved image plates are inked, each in a single color and each ink image is separately transferred to the medium. In most color print systems, images in each of three primary colors together with black, are printed in registration so that the finished picture is a composite image. The color in any incremental area of the finished print is determined by the relative amounts of each primary color present in that incremental area.
To print in the thermal wax transfer process, an individual printing electrode is heated by passing an electrical current through the electrode. A film carrying wax of a single color is placed in intimate contact with a web, conventionally paper, upon whose surface a wax dot is to be deposited. The "sandwich" thus formed is held against the electrode by a roller which acts as both a platen and a heat sink. Where the temperature of the film exceeds the melting temperature of the wax, a small area of wax melts. Additional amounts of heat must be supplied to melt sufficient wax for the creation on the medium of a mark of the desired size. At the cooler print medium, the wax starts to chill and begins to re-solidify.
The medium and ribbon are permitted to remain in contact during travel away from the print head, during which time the re-solidifying wax preferentially adheres to the medium rather than the ribbon. The ribbon is then separated from the web and travels to a take up roll. The web medium travels to the next print station where the printing process is repeated with a wax of a different color.
In printing on a moving web, it is important to determine where a dot is to be deposited. It is also important to determine when the mark is to be deposited. With conventional printers of the prior art, usually the time available for depositing a line of colored dots was sufficient to permit printing whenever it was reasonably certain that the correct printing location for a line of marks had arrived at the print head. When the printing location for the next line of marks reached the print head, the next line was printed.
As the web moves, there are "windows" of opportunity within which a dot row must be printed. If the human eye is to be satisfied with the result, the dots must be aligned in a direction transverse to the direction of travel of the medium and the spacing between adjacent lines must be uniform. Further, and depending upon the subject matter of a document, the alignment and registration requirements may be quite stringent because of the sensitivity of the eye to misalignments, especially in patterns that include straight lines and smooth curves.
It would be desirable to have a relatively high speed (up to approximately 12 inches per second or greater) full color (three primary colors plus black) thermal printer that can reliably and repeatably produce images that accurately represent a "picture". The source of images may be an image file in a computer and result from the manipulation of an image creating computer program. It is equally possible to "scan" color "documents" from a variety of sources into a computer file using presently available scanners and programs. Such "documents" can be printed using a color printer without the need of creating a plurality of engraved printing plates.
Such a printer should have a resolution of up to 600 "dots per inch" (dpi) and greater and be capable of printing upon various media including paper, fabric, plastic film or metallic foils. The color palette should permit a range of colors and hues sufficient for perception by the human eye. Generally a color range of at least 8 shades for each primary color and black, which can be represented by up to 32 data bits should suffice.
So that monochromatic ribbons can be used in the printing process and to obviate the need for reversing web travel between colors, four dedicated printing heads should be serially arranged in the path of the moving web. The printed images from each of the print heads should maintain accurate registration and the row-to-row spacing of adjacent printed rows should remain constant.
According to the present invention, a full color thermal printer capable of achieving print speeds of greater than 6 inches per second utilizes digital computers and a mechanical position sensor to assist in determining not only the "time window" during which the dot row to be printed will be available to the print head, but also the optimum time, duration and magnitude of electrical impulses which are to be applied to the individual electrodes of a thermal print head to effectuate printing of a mark.
A high resolution encoder signals paper travel through the printer to the computer and to a counter. Individual registers are provided for each print head. Within each register is stored a number representing a length of web. A first register holds a number whose value represents a length of web sufficient for acceleration from a rest position to a steady state velocity. In other registers, numbers are stored whose value equals the sum of the first register number plus other numbers whose value represents the distance between printing heads. These numbers are automatically updated with a number whose value represents a correction for thermal expansion or contraction of the entire mechanism.
Each of the registers has an associated comparator whose second input is connected to the counter. When the counter value reaches the value of the number stored in a register, the associated comparator generates an actuating signal which is applied to the associated printing circuits.
According to a preferred embodiment, the several registers are loaded with constants which are stored in the computer and the web is advanced. The constants are precisely corrected to reflect actual distances after test print runs are made to check registration of the printed dot rows. By correcting the constants, each successive print run can begin with more accurate constants stored in the computer.
An alternative method that can be used to generate actuating signals would be to load the constants as a negative number in a counter register to which is applied the pulse output of the encoder. Each register is then counted up to zero. Several alternative embodiments could be designed to achieve a similar result. It is only necessary that the printing commence at a predetermined spatial interval after start up and continue at predetermined intervals thereafter.
In an embodiment in which the resolution of the printing is 300 dots per inch ("dpi"), the design of the precision encoder is such that 9 pulses are generated from one dot row to the next. Accordingly, a selected number, 9, is generated and added to increment the registers. After the counter reaches the incremented number, a new actuating impulse is generated and the register is incremented again. Each ninth pulse establishes the beginning of a "time window" during which printing should be initiated for each dot row.
This result can also be implemented by using a modulo-9 counter once the printing is initiated. Alternatively, the counting registers can be incremented by a count of 9 (minus or plus, respectively) and each zero condition can signal a new print line.
For those incremental areas which are to be printed with more than one color, it is important that the electrodes of the print head for each of the selected colors be heated such that the colored wax liquefies and is deposited directly over the wax dot applied by a prior print head. The magnitude and duration of the electrical impulse to the printing electrode determines the size of the dot of wax which is transferred to the web medium and the precise location that the dot will occupy.
When printing in many colors, the perceived color of any given area is the function of the primary colors (with black) that are employed and the relative size of each area of color that is deposited in the given area. As with the pointillists or with "half tone" or other engraved printing plates, the shade and hue of any color that is created is a function of the density and size of each color component. Using two dots of the same size but different colors will be perceived differently than two dots of the same different colors but of different size. In the preferred embodiment, the printer generates uniform sized dots.
In a preferred mechanization of the present invention, a digital computer is extensively utilized to store and retrieve data files corresponding to documents to be printed. The computer also provides necessary operating controls to the components of the web drive system. The computer determines when and what to load into the various printer buffers and registers that actually supply the print signals to the individual thermal electrodes of the several print heads that are required for a full color printer.
For example, a full color document to be printed is initially stored in a printer memory that has as many memory locations as there are "dots" in the entire document. In one embodiment, there is a "word" in each memory location. For a document that will be 8".times.10", the memory must have at least 2,400 columns and 3,000 rows to print a document at 300 dpi. Each stored word contains a predetermined number of bits in four groups, one for each of the colors to be printed.
In the present example, it has been found preferable to follow the lead of conventional 4-color platen printers which utilize three primary colors and black. In the printer chosen for the present example, the colors selected, in print order are yellow ("y"), magenta ("m"), cyan ("c") and black ("k"). For ease in explanation, components which are particular to each color will be subscripted by the appropriate letter, y, m, c, or k.
Each of the printing heads has associated with it a register which stores a portion of the document. It has been found convenient to store approximately an inch, or 400 lines of the document in the first or yellow register R.sub.y. The second or magenta register R.sub.m stores the same amount of data plus an amount of data which corresponds to the nominal distance between the print heads. Here that distance is about 4" or 1,200 lines for a total of 1,600 lines of magenta color data in the magenta register R.sub.m.
Similarly, the cyan register R.sub.c stores the 400 lines plus the 1,200 lines representing the distance between yellow and magenta and an additional 1,200 lines for the distance between the magenta head and the cyan head. As can be expected, the black register R.sub.k stores a number of lines equal to that stored in the R.sub.c register plus 1,200 lines representing the distance from the cyan head to the black head.
The individual printing heads which are commercially available from suppliers provide both storage and addressing capabilities so that there is a communication protocol that is dictated by the head selected for printing.
As noted earlier, the position encoder causes the generation of periodic printing impulses which are applied to the heads as a strobe pulse opening a time window during which specific energy pulses of preselected magnitude and duration are applied to the individual electrodes that are to be heated to a temperature sufficient to melt wax from the ribbon and to transfer the wax to the web medium.
A problem of hysteresis is presented in a thermal printing process. While the temperature of a printing electrode or nib can be quickly raised by the application of an energy pulse, a longer time is required for the nib to cool, generally along an exponential curve that is affected by the ambient temperature of the print head. At web or media rates of 3 to 6 ips, the nibs require from 5 to 10 print rows or lines to cool to the ambient print head temperature. The rate of cooling can be increased by providing a refrigerated heat sink, such as may be achieved using thermal junction material which has been commercially available for some time.
Because of the way in which the nibs cool down, a nib that has printed in the next prior line (a "hot" nib) will be "warmer" than a nib which has not printed in the prior interval. Further, a nib which has not printed for two prior intervals will be even cooler. Less important, but important nonetheless is the status of the nibs adjacent a subject nib. If the neighboring nibs have printed in the prior line and are hot nibs, thermal conductivity will transfer some of their heat to the subject nib.
The resistance of the individual nib circuits will also have an impact on the ability of the nib to melt the proper quantity of wax at the proper time. Further, each color wax ribbon has its own characteristics including temperature of melting and fluid flow properties. These, too, affect the magnitude and duration of an applied energy pulse and when such a pulse should start and terminate, relative to the passage of the medium over the nib.
Several approaches have been suggested to meet these conditions. One suggestion provides a plurality of thermal energy pulses of varying duration depending on whether a nib is "cold", "warm" or "hot". Another suggestion requires that all applied thermal energy pulses terminate at the same time. Yet a different suggestion requires that all nibs be kept at an elevated resting temperature just below that needed for printing by supplying "maintenance" pulses during every interval that a nib is not actually printing.
An alternative suggestion has a cold nib energized during the prior print interval. By the time that printing should commence, the temperature will have been elevated sufficiently so that a pulse of lesser duration will be adequate for printing. Yet another scheme utilizes binary weighted time intervals for supplying energy pulses to each nib where the longest interval may be one hundred twenty eight times the shortest interval and any particular interval can have a duration of from 4 to 512 microseconds. These suggestions have been mechanized under computer software control in a preferred embodiment of the invention.
All of these suggestions acquire greater importance as printing speeds increase. For example, an increase in print speed from 3 to 6 inches per second halves the time available for nib cool down and interval during which printing takes place. However, the rate at which the wax melts or flows is generally immutable and independent of the print speed. Also, as printing speeds increase, it may become necessary to anticipate and commence printing "early".
In printing text or full color documents where there are areas of solid color or black, it is necessary that the individual dots be substantially square, fully occupying the incremental area allocated to the dot. This is partially accomplished by the width of the individual nibs, which when sufficiently heated, can melt a wax area greater than the width of the nib. The duration and magnitude of the energy impulse determines the "length" of the dot to be deposited. However, the manufacturer has recommended that a nib duty cycle should not be greater than 50% over several print lines, which would contraindicate a continuous energy level at the nibs for printing solid areas based on "cold" nib temperatures.
However, when printing areas that are to be solid colors, it may be more desirable to print dots that are slightly oversized so that should a dot be undersize, there will be no appearance of "borders" around such individual dots. By examining the contents of a document, areas of solid color can be anticipated and the energy impulses supplied to the printing electrodes can be increased so that the dots in the area of solid color can be slightly oversize to assure a smooth overlap.
For ease in discussing the invention and its various embodiments, some conventions have been adopted. The letter "T" has been used to represent temperature, the letter "t" has been used to represent time, the letter "N" has been used to designate printing nibs and the letter ".pi." has been used to represent pulses.
One plan that has been implemented permits a selection of one of "n" intervals where n has a value that can be at least 15. For each line of a given color, the print register will store a "1" where a mark is to be printed and a "0" if no mark is to be printed. A logic unit stores a number corresponding to the last line in which a mark was printed. This is implemented by using a counter which is reset to "1" by each stored "1" and which is incremented by "1" by each stored "0". The counter is limited to a count of n. Print pulses designated .pi..sub.1 through .pi..sub.n, are progressively longer.
It has been determined that it is preferable if the print pulses of all of the nibs end together. Accordingly, the print head is loaded in n sub intervals, each corresponding to a print pulse .pi..sub.1 through .pi..sub.n. The head storage cells corresponding to each nib which is to receive a .pi..sub.n pulse are loaded first, prior to the occurrence of the .pi..sub.n pulse. Next, the cells corresponding to the nibs which are to receive a .pi..sub.n-a pulse are loaded, just prior to the occurrence of the .pi..sub.n-1 pulse. The cells to receive .pi..sub.n-2 and .pi..sub.n-3 pulses are then loaded and finally, the cells corresponding to the hot nibs and which are to receive a .pi..sub.1 pulse are loaded last.
In this scheme, depending upon the implementation, each nib will receive either a continuous pulse lasting through the interval or a series of sequential pulses which will appear to be substantially continuous. An advantage of this procedure is that all nibs that have printed will, at the end of a print interval, be at the desired temperature at the same instant.
Another problem that must be addressed is one of non constant velocity. If velocity varies by more than a few per cent, the cool down time of the nibs will be affected and any regular variation in velocity will become noticeable as a variation in print density of the dot row or line. Since velocity information can be obtained from the encoder, an appropriate routine within the computer can adjust the length of the various strobe print pulses to accommodate the velocity changes.
In summary, determining the magnitude and duration of the electrical impulse that is to be applied to each of the electrodes to produce a colored dot of the desired size and placement, requires the use of algorithms that utilize functions of the prior history of an electrode and the history of electrodes on either side of that electrode, as well.
Other algorithms examine the velocity of the web to correct for velocity variations. Algorithms also consider the overall and regional temperatures of the print head, the characteristics of the web and the colored wax, and the resistance of each electrode in the printing head and even the contents of the as yet unprinted portion of the document file. Other significant parameters include the thermal transfer characteristics of each print head, its heat sink and the printing platen.
In one embodiment, the prior history of a particular electrode over a number of prior print cycles is combined with the history of the next adjacent electrodes for a fewer number of prior cycles and with the history of more remote adjacent electrodes for even fewer prior cycles.
Because the printing process requires a substantially constant voltage across each of the printing electrodes and because the current requirements are a function of the magnitude and duration of the printing impulse that is applied to each of the electrodes, it has been found useful to create, in the computer memory, an estimate of the instantaneous current needs of each of a series of subregions of the print head.
This can be done by examining the document file, or, for that matter, printing buffer registers to note which electrodes will require current and how much current is to supplied to all of the electrodes. One solution is to calculate a "weighting" for each electrode and to modify the duration of the pulse applied to that electrode to be sure that adequate power is supplied for the desired result.
Another approach is to subdivide a printing head into several regions and determine from the total available energy, the energy available in the region and the desired energy to be supplied to each electrode. Yet another approach that could be employed if it appears that insufficient current will be available for a particular line, is to "preheat" some electrodes in earlier lines to reduce the duration and magnitude of the printing impulses sufficient to deposit dots for those electrodes.
For better control of dot size and shape, it may be desired to maintain each electrode at a "resting" temperature which may be fairly close to the temperature at which the wax melts. This enables the use of shorter printing pulses of lesser magnitude which, in turn, enables a closer control of the time that the wax actually melts and transfers to the web medium. Such timing is essential if accurate registration of the colored dots is to be achieved.
Another technique to assure reliable printing at higher speeds would be to initiate the provision of printing current to "cold" electrodes in advance of the time that the "warmer" electrodes are powered. Because the interval between such printing impulses is already subdivided by the position encoder, printing for a particular electrode could be commenced during a prior interval by a count less than 9 or, depending upon the web velocity, may commence during the printing of an earlier line.
It is therefore an object of the invention to provide circuits and systems to superimpose colored dots to form other colors on a moving web.
It is an additional object of invention to provide a thermal printer whose printing electrodes are individually energized by impulses whose magnitude and duration depend upon the nature of the image to be printed.
It is an additional object of invention to provide a thermal printer whose printing electrodes are individually energized by impulses whose magnitude and duration depend upon the velocity of the medium.
The novel features which are characteristic of the invention, both as to structure and method of operation thereof, together with further objects and advantages thereof, will be understood from the following description, considered in connection with the accompanying drawings, in which the preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and they are not intended as a definition of the limits of the invention.